I'll disclaim first that this might not apply to Tolkien, but for literature in general that a good number of vague notions of faith are literary cowardice in the unwillingness to turn it into the tangible. In other words, the author does not want to attempt any greater description for lack of knowledge or experience of it or not wanting to be wrong in the end or offend sensibilities of possibly religious critics.

I have found many a story that are disappointing, for example, because it turns out the evil one is fighting is a minion or agent of the devil - not the devil himself. Why not go for the root? It would be a braver move by the author. If I recall correctly, The Exorcist is guilty of this. If not, others are. One story which is not guilty of such cowardice is Quentin Tarantino's Inglorious Bastards because it not only goes after agents of Adolf Hitler, but Hitler himself, as a fantasy independent of our own reality. That was brave.

It is therefore unremarkable at best to me that one should avoid, for example, Frodo's description of the peoples, life and land of Valinor upon reaching it just as The Scouring of the Shire was an epilogue to the War of the Ring. Perhaps the authors of stories like this simply feel they are not capable or worthy to their own satisfaction that they can imbue the text with that certain emotional and spiritual value. Or they are afraid of it and this idea of leaving things to imagination or faith is just a rationalization.